It means the Met Police believes laws relating to covid restrictions have been broken in some instances.
The revelation puts the partygate scandal firmly back on the political agenda and a renewed spotlight on Johnson’s leadership.
The war in Ukraine over recent weeks had taken some of the heat off the prime minister.
Labour deputy leader Angela Rayner said: “The culture is set from the very top. The buck stops with the prime minister, who spent months lying to the British public, which is why he’s got to go.
“It is disgraceful that while the rest of the country followed their rules, Boris Johnson’s government acted like they didn’t apply to them.
“This has been a slap in the face of the millions of people who made huge sacrifices.“
Lib Dem leader Sir Ed Davey said: “If Boris Johnson thinks he can get away with partygate by paying expensive lawyers and throwing junior staff to the wolves, he is wrong.
“We all know who is responsible. The prime minister must resign, or Conservative MPs must sack him.”
The allegations led some Tory MPs to publicly call for Johnson to go and some confirmed that they had submitted a letter of no confidence in his leadership to the 1922 backbench committee of MPs.
Labour leader Keir Starmer also demanded the prime minister quit, but later put that on hold, saying it was time for politicians to unite over the war in Ukraine.
However, only last week senior Tory MP Mark Harper warned that partygate “hasn’t gone away”.
Education minister Will Quince refused to say in a round of broadcast appearances this morning whether the PM should quit if he is fined.
However, he did admit that restriction-busting parties “shouldn’t have happened”.
The Met said on Tuesday: “The investigation into allegations of breaches of Covid-19 regulations in Whitehall and Downing Street has now progressed to the point where the first referrals for fixed penalty notices [FPN] will be made to Acro Criminal Records Office.
“We will today initially begin to refer 20 fixed penalty notices to be issued for breaches of Covid-19 regulations.
“The Acro Criminal Records Office will then be responsible for issuing the FPNs to the individual following the referrals from the MPS.
”Detectives launched their investigation at the end of last year after reports emerged of gatherings taking place during the pandemic.
The Met announced last week that more than 100 questionnaires had been sent out to people at the reported gatherings.
They included the PM and chancellor Rishi Sunak.
However, police said they would not confirm the number of referrals made from each individual event it is investigating as providing this breakdown at this point could lead to the individuals involved being identified.
Johnson was forced to apologise in January following the publication of senior civil servant Sue Gray’s report into the scandal, which blamed “failures of leadership and judgment” in Number 10 and the cabinet office for the affair.
